1. Field of the Invention
The present invention pertains to the determination of the location of subsurface reflectors and more particularly to the development of a suite of constant travel time curves defining an envelope curve that is sharply focused at the true interval velocity, thus permitting determination of the reflector location.
2. Related Prior Art
The determination of the location of subsurface reflectors has been a continuous endeavor in the search for hidden hydrocarbons, often known as oil exploration. In this endeavor, many methods have been associated with the processing of seismic or acoustic waves to approximate the location of elusive subsurface interfaces. Among these methods are the determination of the average subsurface velocity, or the average of a plurality of subsurface interval velocities, i.e. the velocity between interfaces or reflectors. With an estimate of the average velocity, a time and depth correlation is achieved to approximate the location of subsurface reflectors or density changes. Although the determination of the location of subsurface reflectors is complicated by dipping formations, multiple reflections between interfaces, refraction of acoustic waves and other non-linear events, the determination of reflector depth has its basis in velocity determination.
Some examples of prior art methods that are used to determine subsurface acoustic velocity are as follows. All of these methods compute the velocity by iterative or repetitive means.
U.S. Pat. No. 3,611,278 titled "Interval Velocity Determination" (Guinzy et al) relates to the conversion of a suite of seismograms to values of interval velocity and dip for each subsurface layer encountered in geophysical exploration. An iteration process fits an arbitrarily dipping Snell's Law layered model to the observed field seismograms. Use of the velocity model permits a migration of original data into its true spatial positions.
A prior art method for determining the interval velocity in an area between reflectors is discussed in U.S. Pat. No. 4,110,729 which is titled "Reflection Seismic Exploration: Determining Interval Velocity in a Subsurface Layer" (Vreugde). This patent relates to a reflection seismic exploration system which reduces the influence of near surface irregularities, without knowing or measuring these irregularities. This system determines the velocity of a seismic signal between two reflectors, the normal travel time in the layer between the reflectors, and the thickness of the layer. Two consecutive shots, spaced apart along the surface, are fired into the same spread of detectors. Three different time-distance relationships are recorded. First, the relative arrival times of the reflections from each of the two reflectors at each of an array of detectors for one of the shots, and the shot-detector distances along the surface are recorded. Second, the difference between the relative arrival times of the reflections of the two shots from the shallower reflector at each detector and the distances between each detector and the midpoint between the shots is recorded. And, finally, the difference between the relative arrival times of the reflections of the two shots from the deeper reflector at each detector and the distances between each detector and the midpoint between the shots is recorded.
The first, second and third time-distance relationship are curve-fitted to defined expressions to find the values of selected coefficients of these expressions. The coefficient values are combined to find the normal interval time and the interval velocity in the layer between the two reflectors, which are indicative of the nature of the layer. This is done to determine the thickness of the layer.
Another method for determining the interval velocity between reflectors is discussed in U.S. Pat. No. 4,571,710 titled "Seismic Method for Identifying Low Velocity Subsurface Zones" (Neidell et al). This patent is primarily concerned with the determination of zones between reflectors which have a velocity below a predetermined maximum. Neidell et al. relates to a method of producing a two-dimensional display from exploration seismic data designed to indicate zones of anomalous low velocity in the subsurface. Such zones may be indicative of porosity and the possible occurrence of hydrocarbons. These zones are localized in terms of position along the seismic profile and in the approximate zone of reflection arrival time. Data used in making the display is derived from both the common depth point (CDP) stacked seismic profile and corresponding velocity analyses used to stack the data. Stacking velocity curves are plotted according to CDP location for each reflector of interest. These curves are overlain in pairs using calibration calculations. Calibration is used to reduce noise and compensate for velocity variations resulting from changes in separation or dip of the two reflectors from which the overlain velocity curves are derived. It is presumed that the interval bracketed by each reflector pair taken in turn has uniform or only regional lateral variations of interval velocity. Convergence of the paired curves indicate zones of either high or low interval velocity of local nature. A display of the velocity curves for all possible reflector pairs and vertical bands of color which are assigned to identify the reflection interval of particular low velocity zones may be provided. Horizontal position is indicated by the CDP location.
U.S. Pat. No. 3,794,827 titled "Invel System of Velocity Determination" (Widess) relates to a method of compositing multifold common-depth-point data from seismic prospecting operations to improve the making of static corrections, particularly when determining velocity. These static corrections are used to eliminate time differences in arrival of reflected seismic events on the various traces of a seismic spread due to differences in thickness of the low velocity or "weathered" layer below the geophones. Seismic waves are generated successively at generating points, each near the earth's surface. Seismic waves are received and reproducibly recorded at geophones at least one of which is close to the generating point and another is near the location of another generating point. Reproduced reflected waves from each generating point received at the respective near geophone locations are composited at approximately equal peak amplitudes. This procedure is then repeated for new generating and receiving points which maintain the mean location of reflection points on the seismic reflecting beds. Visual traces equivalent to the composited short traces are reproduced after elimination of the normal moveout correction. The reflections from a common reflecting bed are aligned by introducing a static correction into each composited trace to produce substantial time alignment at a mean reflection time. The identical seismic static correction is then applied to any further record processing of the seismic data from the far geophones, similarly composited, before producing visual traces of such data.
U.S. Pat. No. 3,638,177 titled "Method and Apparatus for Seismic Signal Velocity Determination" (Lindblade et al.) relates to a method for determining actual velocities of seismic events occurring on plural seismic traces by deriving a trace coherence factor which varies directly with signal velocity. The method consists of processing move-out-corrected, plural seismic traces at a plurality of different time delay per trace relationships to establish a coherence factor at each time delay per trace. The coherence factor is determined for each probable velocity by summing all trace values and then finding a respective difference value between each of the trace values and the sum value. The difference values are further processed by summation of their absolute values for derivation of a reciprocal trace value. The reciprocal trace value is then utilized as a gain control factor for controlling time variation of the amplitude of summed seismic traces. The output trace value controlled in this manner constitutes an accurate indicator of coherence of events occurring at the selected velocity.
U.S. Pat. No. 2,276,335, titled "Method of Making Weathering Corrections" (Peterson) relates to a method for determining the depth of the weathered layer of the earth. Also included in this patent are methods for determining the wave velocity in the weathered layer once the depth is determined by combining this determination with the vertical two way travel time. In addition, this patent contains a discussion of a method for determining the acoustic velocity in the high speed layer located directly beneath the weathered layer.
A typical example of the use of velocity determination for vertical seismic profiling may be found in U.S. Pat. No. 4,627,036, titled "Vertical Seismic Profiling" (Wyatt). In Wyatt, segments of vertical seismic profile data are mapped into surface seismic data time. By this transformation, data which is more easily interpreted with respect to determining subsurface structures is provided. The method presented in Wyatt uses subsurface velocity to convert seismic data obtained from receivers located downhole to data resembling conventional seismic data.